allow validEndDate property value for each feature to have the value 'null'

[cDanowski]

I noticed that when declaring feature-wise property values for validStartDate and validEndDate import/update requests get rejected when validEndDate property value is null for one or more feature.

Actually the KomMonitor Data Management API allows validEndDate to be null on both, the global validEndDate as well as the feature-wise property.

I made a quick fix using a try catch block in commit 6b49849

You may revise it and suggest/implement a better solution.

Regards,
Christian